A conventional die assembly A shown in FIG. 6 includes an upper die U and a lower die L, and the upper die U is connected to the lower die L via die-set guides so as to move backwards and forwards (moves upwards and downwards).
The upper die U includes a punch holder Ua, a punch plate Ub and punches Uc and is mounted on a press ram R of a press, while the lower die L includes a lower die base La and a die Lb and is mounted on a bolster B of the press.
A stripper plate S is provided between the upper die U and the lower die L so as to move backwards and forwards via (moves upwards and downwards) via the die-set guides, and in the die assembly A described above, a thin plate (a material for punching) is pressed against the lower die L by the stripper plate S, and the thin plate is punched by the punches Uc of the upper die U by lowering the upper die U towards the lower die L.
Here, in the conventional die assembly A described above, in the event that the press ram R is slightly inclined or deflected or moves sideways due to wear produced in association with an eccentric load at the time of punching or usage, there is caused a deviation in a positional relationship between the upper die U mounted on the press ram R and the lower die L.
This makes a clearance between the punches Uc of the upper die U and the die Lb of the lower die L uneven, which may cause a fear that not only the generation of a punching failure may occur, but also damage to the punches Uc and the die Lb may also occur.
Since a pilot pin (not shown) which is provided on the upper die U is also caused to move, there is caused deterioration in guiding the pilot pin into a pilot hole (not shown) provided in the lower die L.
These phenomena lead to serious problems with respect to the die assembly for use in punching a thin plate whose thickness is extremely thin, in precision punching which requires a small clearance between the punches and the die and in punching a product which requires severe dimension and shaping accuracies.
Then, with a view to solving the aforesaid problems, there is provided a die assembly in which a stripper plate for guiding punches when punching a material for punching are provided on a lower die so as to move backwards and forwards (refer to Patent Document 1: JP-A-2001-47147).
In a die assembly A shown in FIG. 7, a stripper plate S is installed between an upper plate U and a lower plate L, and guide rods Sg, Sg which extend downwards are provided on the stripper plate S.
These guide rods Sg, Sg fit in guide guide bushings Lc, Lc provided in the lower die base La so as to rise and fall, and the stripper plate S is provided so as to move backwards and forwards (moves upwards and downwards) relative to the lower die L by the guide rods Sg, Sg being guided by the guide bushings Lc, Lc.
In addition, the punches Uc, Uc on the upper die U fit in the stripper plate S so as to move backwards and forwards (moves upwards and downwards), whereby the stripper plate S guides the punches Uc, Uc on the upper die U at the time of punching. Further, the stripper plate S also guides a pilot pin (not shown) provided on the upper die U.
According to the configuration of the aforesaid die assembly A, due to the punches Uc on the upper die U being guided by the stripper plate S which is not connected to the press ram R, the positional relationship between the upper die U and the lower die L is maintained without being affected by the inclination or deflection or sideways movement of the press ram R.
Consequently, the generation of a punching failure or damage to the punches/die can be prevented which would otherwise be caused by the inclination or deflection of sideways movement of the press ram R, and even with a thin material for punching, punching with good dimension accuracy and shaping accuracy is enabled.    Patent Document 1: JP-A-2001-47147